1. Field of the Invention
The present invention relates generally to systems, kits and methods for joint replacement using multiple components. In one embodiment, the present invention includes as components a head, a neck and a stem.
2. Related Art
Artificial joint prostheses are widely used today, restoring joint mobility to patients affected by a variety of conditions, including congenital, degenerative, iatrogenic and traumatic afflictions of the joints. The satisfactory performance of these devices can be affected not only by the design of the component itself, but also by the surgical positioning of the implanted component and the long-term fixation of the device. Improper placement or positioning of the device can adversely affect the goal of satisfactorily restoring the clinical bio-mechanics of the joint as well as impairing adequate fixation of the component when implanted within the medullary canal of the bone. The primary role of an implantable joint prosthesis is to restore the extremity distal to the diseased and/or damaged joint to normal function.
As one example, an implantable joint prosthesis can be used to provide an artificial hip. When the prosthesis is situated in this position, significant forces such as axial, bending, and rotational forces are imparted to the device. The prosthesis must endure these forces while remaining adequately fixed within the medullary canal, because adequate fixation is necessary to ensure the implant's proper functioning and a long useful life. Early designs of artificial hip components relied primarily on cemented fixation. These cements, such as polymethylmethacrylate, were used to anchor the component within the medullary canal by acting as a grouting agent between the component and the endosteal (inner) surface of the bone. While this method of fixation by cement provides immediate fixation and resistance to the forces encountered, and allows the surgeon to effectively position the device before the cement sets, it is not without problems. Over time the mechanical properties and the adhesive properties of the bone cement degrade; eventually the forces overcome the cement and cause the components to become loose due to a failure at the cement/bone or cement/stem interface. Alternative approaches to address the issue of cement failure include both biological ingrowth and press-fit stems.
Stems designed for biological ingrowth typically rely on the bone itself to grow into a specially prepared surface of the component, resulting in firmly anchoring the device within the medullary canal. A shortfall of this approach is that, in contrast to components that utilize cement fixation, surfaces designed for biological ingrowth do not provide for immediate fixation of the stem because it takes time for the bone to grow into the specially prepared surface. Press-fit stems precisely engineered to fit within a surgically prepared medullary canal may or may not have specially prepared surfaces and typically rely on an interference fit of some degree of the component within the medullary canal of the bone to achieve stable fixation. Some stem components incorporate sharp cutting flutes or ridges at or near the tip of the stem of the component which are designed to engage in the cortical bone and tend to lock the device in place. A problem with this approach is that once the flutes engage, the rotational alignment of the device is determined.
It is therefore desirable to provide a component which allows adequate rotational alignment and positioning before final placement while achieving sufficient locking for permanent fixation upon full insertion of the device. There is also a need to de-couple the engagement or spacing portion of the stem with the final positioning of the head center, to allow for both optimal positioning and secure engagement to be achieved, independent of each other. The positioning of the device, including the location of the head center relative to the medullary stem portion, effects the biomechanics of the joint. More optimal positioning results in a more efficient joint, and thus lower forces on the device.
By ensuring proper placement of the component one can provide the means for proper functioning and a long useful life of the artificial hip component. Proper placement enables proper function. Placement is typically performed by the surgeon after preparing the medullary canal of the bone to receive the component. Often a surgeon will use a trialing system before implanting the actual component to ensure that the fit and placement is optimized. A trialing system is comprised of trials which are similar in geometry to the actual component and used to assess fit but are intended to be removed, and thus they are usually undersized from the actual component. Alternative methods of trialing to determine predicted fit and proper placement rely on a broaching system. The broaching system is the set of cutting instruments which prepare the medullary canal. Often the broaching systems are designed to perform the trialing function in addition to the bone preparation function by the incorporation of features allowing this dual purpose.
Even though the surgeon goes through extensive preparation to ensure that the ultimate placement of the device will be proper by use of a trialing system, the final placement is dependent on the hand insertion by the surgeon of the implantable component. In cementless components delaying the rotational engagement/alignment of the implantable component before full insertion allows for and can produce more optimal positioning because as the surgeon inserts the device further, optimal placement/position becomes more readily apparent. Additionally, due to the curvature of the femur, the device being inserted tends to follow the curvature as one inserts it, which can cause the device to rotate into a less desirable position. Allowing the surgeon to correct for this rotation before engaging the device fully will also result in more optimal placement. Thus it would be beneficial to have a means for or a device which satisfactorily addresses this issue of obtaining optimal rotational alignment and positioning in a reliable manner.
In cemented components, the problem is somewhat relieved as there is more flexibility in rotational alignment in that final adjustments can be made during insertion before the cement has set. However, it is desirable for movement to be minimized during the cement curing stage as stem movements in the curing cement will almost certainly negatively affect stem-cement attachment. When dealing with cemented components it is important that there be means to allow the stem to remain centrally located within the cement in the medullary canal.
Another problem with prostheses designed for press-fit and/or bone ingrowth fixation is that, in order to achieve adequate stability, the device must make direct contact with cortical bone in the diaphysis, rather than having a layer of intervening bone cement with a lower elastic modulus than that of the stemmed portion of the device. Some components incorporate one or more slots in the distal end of the stem that are designed to increase the stem flexibility, and thus make insertion of the stem easier, and reduce the stresses at the distal tip of the stem in vivo. These slots, however, decrease the strength of the stem relative to a stem with no slot, especially for larger diameter stems that require either a very large single slot or multiple slots to sufficiently reduce the stiffness of the stem. As a consequence, the larger diameter stems are generally more stiff, or weaker, or both, than desirable.
Another consideration related to, but beyond the rotational alignment and positioning of the stem of the device, is that of the resulting head position. The positioning of the device, including the location of the head center relative to the medullary stem portion, affects the biomechanics of the joint. More optimal positioning results in a more efficient joint, and thus lower forces on the device. It is therefore desirable to provide a component that de-couples the engagement or spacing portion of the stem with the final positioning of the head center, to allow for both optimal positioning and secure engagement to be achieved, independent of each other.
The hip head center of rotation is determined by the head position because typical hip heads are spherical. In most devices the head position is determined by the stem position because the two are connected through an integral neck. Many devices in existence use modular hip heads to increase or decrease neck length, which alters both head height and head offset proportionately and simultaneously. The neck portion of the device that is attached to the stem receives the modular heads. This results in the head position being integrally linked and thus aligned with and determined by, the stem portion.
Multiple positions of the heads may be achieved by using hip heads with various bore dimensions and extended or reduced offsets or skirts which limit the positioning of the head to the angled neck axis. In many instances this may not be appropriate as one may only wish to increase offset while maintaining head height (or vice versa), which can not be accomplished with the modular head type devices previously described. In addition, one could not address anteversion of the head in such a device as described. The amount of anteversion is determined by the angular difference between the stem-axis/neck-axis plane to that of the coronal plane. Since the head position is directly linked to the stem position, anteversion can only be achieved by sacrificing stem position by rotating the stem. Thus it would be beneficial to be able to achieve variable positioning of the hip center of rotation independent of stem position as well as independent of both head offset and head height. The ability to de-couple hip head center of rotation from stem positioning would be an additional benefit because it would allow for more freedom in stem placement independent of neck axis placement to achieve the desired anteversion. It is desirable to provide a modular system for joint replacement that provides for this anatomic flexibility.
To conform to a variety of anatomic configurations, devices may incorporate modular components, such as modular stems with modular sleeves, or modular proximal and distal portions of the stem to provide some degree of adjustability for the final stem geometry. This adjustability may or may not include lateral offset, leg length, and/or anteversion, depending on the specifics of the design and on the available components. Such devices have used two basic means of connection, tapers and threads, used alone or in combination. Taper connections have the disadvantage that the final axial position of the two components, relative to each other, is dependent on the precise geometry of the tapers; deviations in geometry within the tolerances allowed for manufacturing results in deviations in the final axial position of the modular component with the tapered connection. The strength of the coupling between the components with the tapered connection is also in part dependent on the level of force used to assemble the components. Similarly, threaded connections have the disadvantage that the strength of connection is in part dependent on the magnitude of torsion applied to the threaded coupling mechanism during assembly. Insufficient impaction force for tapered connections, and/or insufficient torsion for threaded connections, applied during assembly can leave the assembled component at risk of disassembly during the functional lifetime of the device. Unintended disassembly of implanted components is a serious complication that generally requires medical intervention ranging in severity from closed manipulation to surgical revision. This can be a significant risk for tapered and/or threaded coupling means especially considering that the assembly is accomplished in the operating room, rather than under more controlled conditions such as in a factory, in order to take full advantage of the modularity. Thus a design that provides a coupling means for the modular components that has a more reproducible final geometry and reproducible strength of connection, that is less dependent on the surgeon, operating room staff, or other persons acting outside the place of manufacture, would be of significant benefit.
It is furthermore desirable to provide a modular system for joint replacement whose components are easy to assemble in an operating room setting. In certain devices, adjustability is achieved at the expense of simplicity. A complex apparatus with a large number of complicated pieces may require laborious assembly, often while the patient is under general anesthesia. Time spent in selecting and arranging component parts of a complex reconstructive device may add significantly to the patient's anesthesia exposure. In addition, a system with a number of component parts provides for the surgeon a steeper learning curve and a less intuitive feel for the relationship between the implant and the joint being reconstructed. Keeping a modular system simple while still providing valuable anatomic variability would be a significant benefit for doctor and patient alike.